OK, up front - I don’t know anything about “usbwebserver”. Hopefully that doesn’t matter, but I’m sure if it does someone will chime in.
Most often, this confusing error just means that the remote server wasn’t talking SSL (or TLS) after all.
Check you haven’t done anything silly like, tell an HTTP (not HTTPS) server to use port 443, or ask for an HTTPS connection on port 80.
For the certificate with the name www.dekaaters.be to be used and working with your local PC you first need to have the certificate from the web site (not a problem) but also the Private Key which will be a file stored with the web site configuration. I don’t know if your downloaded site will include this, it would likely be named privkey.pem or something very similar. Don’t show this to us! You need it for this to work, but you mustn’t show anybody what’s inside it, hence “private” key.
The site on your local PC needs to be configured to use that key and certificate (which usbwebserver might do automatically, but I have no idea) otherwise to help we’d need to know the type of web server (e.g. Apache, nginx) and so on.
Once the site is configured right, your web browser needs to believe it’s talking to www.dekaaters.be, after all that’s the name on the certificate. If the name you type into the browser and the name on the certificate don’t match, that’s a critical security error. You might be able to do this using the “hosts” file but it’s really beyond the scope of this site.